P0.1: scaffold m3te sx project from the proven game structure

Stand up build.sx (macos + ios/ios-sim targets, bundle id
co.swipelab.m3te, output sx-out/ios/M3te.app, assets dir) and a minimal
main.sx that brings up the platform (UIKit+Metal on iOS, SDL3+GL on
macOS) and renders a solid-clear frame. Add assets/ and goldens/
directories and a .gitignore for build artifacts.

Modeled on game/build.sx and game/main.sx; modules resolve from the
compiler binary with no -L flag.

build.sx

@@ -0,0 +1,35 @@
+#import "modules/compiler.sx";
+#import "modules/platform/bundle.sx";
+
+configure_build :: () {
+    opts := build_options();
+    opts.set_bundle_id("co.swipelab.m3te");
+    opts.set_post_link_callback(bundle_main);
+    if OS == {
+        case .macos: {
+            opts.set_output_path("sx-out/macos/M3te");
+            opts.set_bundle_path("sx-out/macos/M3te.app");
+            // Ad-hoc sign (empty identity) so a local `open` launch isn't
+            // Gatekeeper-rejected. SDL3 comes from Homebrew.
+            opts.add_link_flag("-L/opt/homebrew/lib");
+            opts.add_link_flag("-lSDL3");
+            opts.add_asset_dir("assets", "assets");
+        }
+        case .ios: {
+            opts.set_output_path("sx-out/ios/M3te");
+            opts.set_bundle_path("sx-out/ios/M3te.app");
+            // Device-only: the simulator is ad-hoc signed by the bundler,
+            // which also grants get-task-allow for lldb. Embedding a device
+            // identity / profile in a sim build blocks install + launch.
+            if opts.is_ios_device() {
+                opts.set_codesign_identity("Apple Development: Alexandru Agrapine (DC8VVHJ9W4)");
+                opts.set_provisioning_profile("/Users/agra/Downloads/SwipeS32DevProfile.mobileprovision");
+            }
+            opts.add_framework("UIKit");
+            opts.add_framework("OpenGLES");
+            opts.add_framework("QuartzCore");
+            opts.add_framework("Metal");
+            opts.add_asset_dir("assets", "assets");
+        }
+    }
+}